1. Technical Field of the Invention
The present invention relates generally to the field of planar optical waveguides. More particularly, the invention relates to planar optical waveguide apparatus that include an alignment structure to facilitate measuring the position of an optical waveguide of the apparatus; and to methods for fabricating planar optical waveguide apparatus.
2. Description of Related Art
Planar optical waveguides include a core, comprising one or more channels of transparent material, typically glass, embedded in a cladding of another optical material, typically also glass, that has a refractive index lower than that of the core material. The difference in refractive index between the core and the cladding allows light to be guided in the core.
In many applications that utilize a planar optical waveguide apparatus, it is necessary to connect an external component, such as a laser, a detector, optical fibers, or the like, to the apparatus in such a manner that the component is correctly aligned with the input or output edge of an optical waveguide in the apparatus so as to properly couple light into or out of the waveguide.
A variety of techniques have been developed and are utilized in the prior art to achieve this alignment. For example, one frequently used procedure is an active alignment procedure wherein the component being aligned is moved relative to the planar optical waveguide apparatus while the coupled power between the component and an optical waveguide in the apparatus is monitored. Alignment is optimized when the coupled power is at a maximum value; and, at that time, the component and the planar optical waveguide apparatus are connected together.
While an active alignment procedure can be effective in many applications, it is not always practical, or even possible, to connect the component to the planar optical waveguide apparatus while light is going through the system. For example, if the component being aligned is a laser, and if the planar optical waveguide apparatus will be subjected to a high temperature during soldering of the component to the apparatus; the laser cannot be turned on during the soldering process.
Another common alignment procedure involves measuring the position of the input or output edge of the optical waveguide relative to the position of the component that is to be aligned with the optical waveguide, and then setting and fixing the position of the planar optical waveguide apparatus to the desired values. This procedure, however, requires that the position of the input or output edge of the optical waveguide be precisely measured in x, y and z directions; and these measurements are normally rather difficult to achieve.
In particular, FIG. 1 is a top plan view and FIG. 2 is an end plan view schematically illustrating a planar optical waveguide apparatus that is known in the prior art. The apparatus is generally designated by reference number 10 and comprises a substrate 11, a lower cladding layer 12, a core layer 13 and an upper cladding layer 14. The core layer is formed from a suitable transparent optical material, usually glass; and the cladding layers are formed of another optical material, also usually glass, having a lower refractive index than that of the core layer material. The substrate often comprises silicon.
The difference in refractive index between the material of the core layer and the cladding layers permits light to be transmitted through the core layer; and by forming the core layer 13 into a waveguide, as illustrated in FIGS. 1 and 2, it is possible to guide light through the apparatus.
With reference to FIG. 1, optical waveguide 13 extends through apparatus 10 from input end 15 thereof to output end 16 such that light entering the waveguide 13 at input end 15 is guided through the apparatus and exits the waveguide at output end 16. The light may be input into the waveguide 13 from a first external component, generally designated by reference number 17; and may be output from the waveguide 13 to a second external component, generally designated by reference number 18. The external components 17 and 18 may be connected to the apparatus 10 as schematically illustrated at 19 in FIG. 1.
In order for the planar optical waveguide apparatus 10 to properly transmit light from component 17 to component 18, input and output edges 15a and 16a of the optical waveguide 13 must be precisely aligned relative to the components 17 and 18, respectively; and to achieve proper alignment, it is necessary that the positions of the edges 15a and 16a of the waveguide 13 be accurately known in x, y and z directions.
The x-position of the optical waveguide is the position of the optical waveguide in the x-direction illustrated by arrow x in FIGS. 1 and 2; i.e., the lateral position of the optical waveguide. The y-position of the optical waveguide is the position of the waveguide in the y-direction illustrated by the arrow y in FIG. 1; i.e., the direction perpendicular to the input or output edge of the optical waveguide and that defines the spacing between the input or output edge of the optical waveguide and the component being aligned therewith. The z-position of the optical waveguide is the position of the waveguide in the z-direction illustrated by the arrow z in FIG. 2; i.e, the height of the optical waveguide.
By knowing the position of the external component 17 or 18, and by knowing the x, y and z positions of the optical waveguide, the planar optical waveguide apparatus and the component can be properly positioned relative to one another.
The position of the external component 17 or 18 is determined utilizing suitable indicia provided on the component. The x and y and z positions of the edges of the optical waveguide are determined by detecting and measuring the positions of features of the optical waveguide itself. Specifically, to measure the x and y positions of the edges of the optical waveguide, an image of the optical waveguide is made in a plane of the waveguide apparatus. Inasmuch, however, as the difference between the refractive index of the core material forming the waveguide and the refractive index of the cladding material is normally quite small; attempts to image the waveguide result in a low contrast image, making precise measurements difficult. The z position of the edges of the optical waveguide are usually measured using light reflected from either the upper or lower surface of the waveguide. Again, since the index difference between the core and cladding materials is quite small; very little light will be reflected from the surface, making it difficult to accurately measure the z position of the edges of the waveguide.
What is needed is a method and apparatus that facilitates accurate measurement of the x, y and z positions of the input or output edge of the optical waveguide in a planar optical waveguide apparatus.
The present invention provides a planar optical waveguide apparatus that includes alignment structure to facilitate measuring the position of an optical waveguide of the apparatus.
A planar optical waveguide apparatus of the present invention comprises a core layer and a cladding layer, the core layer including at least one optical waveguide. In addition, the apparatus includes an alignment structure that is spaced from and positioned with respect to the at least one optical waveguide to facilitate measuring a position of the at least one optical waveguide.
A planar optical waveguide apparatus according to the present invention addresses the difficulty in the prior art of accurately measuring the position of an optical waveguide in the apparatus. According to an embodiment of the invention, the position of an optical waveguide in a planar optical waveguide apparatus can be accurately measured by providing alignment structure in the apparatus that is spaced from and precisely positioned with respect to the waveguide The present invention permits the position of an optical waveguide to be measured more easily and with greater accuracy to, in turn, permit an input edge and/or an output edge of the optical waveguide to be more accurately aligned relative to an external component.
According to one embodiment of the invention, the alignment structure includes a first alignment structure to facilitate measuring a z position of an optical waveguide of the apparatus; and a second alignment structure to facilitate measuring x and y positions of the optical waveguide in a plane of the apparatus. The first alignment structure preferably comprises a reflecting member positioned with respect to the upper or lower surface of the optical waveguide, and capable of reflecting sufficient light to permit the z position of the waveguide to be accurately measured. The second alignment structure preferably comprises alignment marks in registration with the optical waveguide and capable of providing a high contrast image to permit the x and y positions of the waveguide to be accurately measured. Because the alignment structure is spaced from the at least one optical waveguide, the structure will not, in any way, interfere with the light transmission characteristics of the waveguide.
The reflecting member may comprise a thin layer of a reflective material, such as a layer of metallic material. Alternatively, the reflecting member may comprise a thin layer of a material, such as silicon, having a high index of refraction relative to the indices of refraction of the core layer material and the cladding layer material so as to define a surface from which sufficient light will be reflected. The alignment marks may include patterns formed in the reflecting layer so as to be easily visible in an image of the optical waveguide.
According to further embodiments of the invention, methods are provided for fabricating a planar optical waveguide apparatus having an alignment structure therein. In general, the methods may include simultaneously forming an optical waveguide of the apparatus and at least a portion of the alignment structure during a single processing step to ensure accurate registration therebetween. Preferably, the single processing step comprises simultaneously forming the optical waveguide and at least a portion of the alignment structure in a single etch process step.
Furthermore, the invention provides embodiments with other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings.